The invention relates to a device and method for precise, fast and continuous tuning of a semiconductor external cavity laser, where the inherent limitations due to friction and mechanical wear are reduced to an absolute minimum.
For applications where a precise and wavelength tunable laser light source is needed, one is often required to use an external cavity stabilized semiconductor laser. A conventional laser comprises an optically amplifying medium between a first and a second reflecting surface. For a monolithic semiconductor laser these reflective surfaces are usually the end facets of the optically amplifying semiconductor chip. A fixed wavelength monolithic laser can be made tunable by providing an Anti-Reflection (AR) coating on the second semiconductor chip facet and arranging for an external movable second reflecting surface to provide wavelength selection.
A plurality of methods exist for effecting wavelength selectivity, where movable diffraction gratings or movable mirrors acting on stationary gratings are the most common, since they typically provide a large tuning range. There also exist several methods for accessing the laser light propagating between the first and the second reflecting surface through the optically amplifying medium, where a partly reflecting and partly transmitting first reflecting surface is one example. Another method takes advantage of the partly reflected light from the above said stationary grating.
Wavelength selection in the semiconductor external cavity laser and thereby wavelength selection of the output light from the said external cavity laser, is usually accomplished by changing the angle between the collimated beam, which originates at the AR coated semiconductor chip facet, and the surface normal of the diffraction grating. The collimation is normally obtained with a lens arrangement.
The change of incident angle can be accomplished either by moving the diffraction grating itself or by changing the beam direction towards the diffraction grating by moving an auxiliary mirror. In both arrangements, continuous tuning is obtained by coordinating the translation and rotation of the moving part.
The movement of the auxiliary mirror or the diffraction grating, to obtain wavelength variation, is commonly achieved using a mechanical drive in combination with a gear device. One such arrangement is described in U.S. Pat. No. 5,491,714.
It is also well known to use electrical motors in external cavity lasers in order to change the wavelength of the light. These arrangements may typically include worm gear or other types of mechanical gear devices.
A disadvantage of using conventional motors with mechanical drives or gear devices in external cavity lasers, is that such mechanical devices prohibit simultaneous fast and accurate tuning due to the inherent friction. One limitation in particular, results from the inherent compromise when choosing a gear ratio for either high accuracy giving low speed, or alternatively, for high speed giving low accuracy.
Among additional disadvantages in known systems for tuning the wavelength of the light in external cavity lasers are for example mechanical wear, friction generated heat, play, variations in lubrication film thickness and lack of mechanical stiffness which are detrimental to the long-term reliability and system performance.
The problem which is solved by the present invention is the difficulty to vary, at high speed and accuracy, the orientation and, if appropriate, the position of the optical elements which determine the wavelength of the light in the external cavity laser. The invention also makes it possible to vary the wavelength with a minimum of friction generated heat and a minimum of mechanical wear.
The above described problem is solved with this invention by means of a method and a device for tuning the wavelength of the light in an external cavity laser comprising an optically amplifying semiconductor chip, a first reflecting surface, an AR coated semiconductor chip facet and a diffraction grating on which at least part of the beam originating from the AR coated semiconductor chip facet is incident and diffracted back to the optically amplifying semiconductor chip, means for collimating the light emitted from the AR coated semiconductor chip facet towards the diffraction grating, and a movable part by means of the movement of which said part the wavelength of the light can be tuned in the external cavity laser, wherein the movable part of the external cavity laser exhibits a rotational movement relative to the optical axis of the external cavity laser, the optical axis being defined by the center of the beam propagating between the first and the second reflecting surface, said movement being actuated by an electrodynamic force generated within an integral section of said movable part.
By virtue of the fact that the movement of the movable part in the external cavity laser is brought about by an electrodynamic force being produced in an integral section of the movable part, the electrodynamic force can be used to directly influence the movement, and thus the wavelength of the light in the external cavity laser. In this way, there is no longer any need for intermediate gear devices and alike. As a result the movable part of the external cavity laser is made to move with a minimum of friction, and its movement and thus the wavelength of the light in the external cavity laser can be tuned precisely, fast and continuously within a large wavelength range.
In one embodiment of the invention, the movable part of the device consists of a rotatable arm on which the diffraction grating of the device is arranged. In an alternative embodiment of the invention the device can also comprise an auxiliary mirror on which at least part of the light diffracted from the diffraction grating is incident and reflected back towards the diffraction grating, in which device said movable part consists of a rotatable arm on which the auxiliary mirror is arranged
The integral section of the movable part where an electrodynamic force is produced, can comprise either a first magnet with the electrodynamic force being produced in interaction with a stationary first coil through which electrical current is passed, or a first coil through which electrical current is passed, with the electrodynamic force being produced in interaction with a stationary first magnet.
In a preferred embodiment, the device also comprise at least one second coil and, associated with it, at least one second magnet, either of the two preferably being arranged on the movable part of the device, the electromagnetic interaction of the second coil and the second magnet being used for detecting movement of the movable part.
When, under the influence of the electrodynamic force, the movable part moves, the second magnet will therefore also perform a movement in relation to the second coil or vice versa, as a result of which a voltage is induced in the second coil. This electromagnetic interaction between the second coil and the second magnet, in other words, the voltage induced in the second coil, is used for detecting the movement of the movable part. The information regarding the movement of the movable part can in turn be used in a control system for improving the control of the wavelength of the light in the external cavity laser.